Fact-checked by Grok 2 weeks ago

Arm

In human anatomy, the arm, or upper limb, is the part of the upper body between the glenohumeral joint (shoulder joint) and the hand. In common parlance, the arm often refers specifically to the upper arm region between the shoulder and elbow, with the forearm extending from the elbow to the wrist, and the hand comprising the wrist, palm, and fingers. The upper limb is a highly mobile structure essential for manipulation, prehension, and interaction with the environment. Each upper limb contains 32 bones: the clavicle and scapula form the pectoral girdle, the humerus is the single bone of the upper arm, the radius and ulna form the forearm skeleton, and the hand includes 8 carpal bones, 5 metacarpals, and 14 phalanges. It features multiple joints, including the shoulder (ball-and-socket), elbow (hinge), and wrist (condyloid), enabling a wide range of movements. The arm is supported by numerous muscles (e.g., biceps brachii for flexion, triceps brachii for extension), nerves from the brachial plexus, and the brachial artery for vascular supply. The upper limbs play critical roles in daily activities, from fine motor tasks to in some animals, and are subject to common injuries and conditions addressed in clinical contexts.

Anatomy

Bones

The arm, or brachium, contains a single known as the , which extends from the to the . This provides structural support and serves as the primary framework for the upper limb's mobility. The is a typical , characterized by a cylindrical shaft and expanded ends, with its overall length varying between approximately 25 to 32 cm in adults depending on sex and stature. At the proximal end, the humerus features a rounded head that articulates with the glenoid cavity of the , forming a key articular surface for movement. Immediately distal to the head are the greater and lesser tubercles, which are roughened elevations separated by the intertubercular sulcus (also called the ); the greater tubercle lies laterally, while the lesser is more anterior and medial. These tubercles are connected to the shaft by the narrower anatomical , and just below this is the surgical neck, a common site of fractures due to its relative weakness. The shaft of the is triangular in cross-section proximally, becoming more cylindrical distally, and features the on its lateral aspect—a V-shaped ridge serving as an attachment site for the . This region also includes foramina for vascular entry and various ridges for muscular attachments. The distal end expands into the capitulum laterally, a rounded eminence that articulates with the , and the trochlea medially, a pulley-shaped structure that fits into the ulna's trochlear notch; these form the primary articular surfaces for the . Flanking the condyles are the lateral and medial epicondyles, which provide leverage points for ligamentous and tendinous attachments. Microscopically, the humerus consists of a thick outer layer of compact (cortical) bone forming the cortex, which provides strength and resistance to bending forces, particularly along the diaphysis (shaft). In contrast, the epiphyses at both ends contain trabecular (spongy) bone, a of interconnected struts that aligns along lines of to optimize weight-bearing and shock absorption while housing . This dual structure enables the to support the arm's weight during activities like lifting, with the compact cortex comprising about 80% of the 's mass in long bones like the humerus. The humerus provides multiple attachment sites for ligaments and muscles integral to its anatomy. Proximally, the greater tubercle offers three facets for the insertion of rotator cuff tendons, while the lesser tubercle and intertubercular sulcus anchor the subscapularis and long head of the biceps brachii, respectively; the coracohumeral and blend into the capsule around the head. Distally, the medial epicondyle serves as the origin for the and , and the lateral epicondyle for the radial collateral ligament and . These bony prominences ensure stable anchorage without delving into soft tissue dynamics. Anatomical variations of the humerus are relatively uncommon but include supernumerary , small accessory bones arising from unfused secondary centers, particularly at the distal end near the epicondyles. For instance, the supratrochlear , a rare variant located dorsal to the trochlea, occurs in less than 1% of the and may mimic fractures on . Other variants involve differences in shape or the presence of a supracondylar process, a bony on the anteromedial distal in about 1-15% of individuals, which can compress nearby neurovascular structures.

Joints

The glenohumeral joint, the primary articulation of the , is a multiaxial ball-and-socket formed by the head of the articulating with the shallow of the . The is deepened by the fibrocartilaginous , a ring-shaped structure that increases the depth of the socket by approximately 50% and enhances joint congruence for improved . Key ligaments include the , which spans from the of the to the greater and lesser tubercles of the , providing anterior-superior reinforcement, and the three (superior, middle, and inferior) that thicken the anterior and limit excessive translation. These static stabilizers, combined with the loose extending from the anatomical neck of the to the glenoid rim, allow for a wide while the and negative intra-articular pressure contribute to overall joint . Synovial structures of the include a lining the capsule, which produces for lubrication, and several that reduce friction between tendons and bony surfaces. Notable encompass the , located between the and tendons; the subscapular bursa, between the tendon of the subscapularis and the ; and the subcoracoid bursa, adjacent to the coracoid. The capsule's anterior and inferior portions are relatively thin, defining anatomical boundaries that permit but rely on ligamentous reinforcements for containment of the humeral head within the glenoid. The elbow joint complex comprises three articulations sharing a common synovial cavity: the humeroulnar joint, a hinge (ginglymus) type allowing flexion and extension; the humeroradial joint, a plane type facilitating gliding; and the proximal radioulnar joint, a pivot type enabling pronation and supination. The humeroulnar component involves the trochlea of the humerus articulating with the trochlear notch of the ulna, while the proximal radioulnar involves the radial head with the radial notch of the ulna. Ligamentous support includes the annular ligament, a strong band encircling the radial head and attaching to the ulna, which stabilizes the proximal radioulnar pivot joint and prevents radial head dislocation. Ligamentous reinforcements at the elbow primarily consist of the medial (ulnar) collateral ligament and the lateral (radial) collateral ligament complex, which provide varus-valgus stability and resist dislocation. The medial collateral ligament originates from the medial epicondyle of the humerus and inserts into the sublime tubercle of the ulna via anterior and posterior bundles, tightening during flexion to counter valgus forces and maintain medial stability. The lateral collateral ligament complex, including the radial collateral and lateral ulnar collateral ligaments, arises from the lateral epicondyle and fans out to the annular ligament and ulna supinator crest, resisting varus stress and posterior subluxation to prevent posterolateral rotatory instability. These ligaments integrate with the joint capsule, which extends from the humerus to the ulna and radius, forming boundaries that enclose the articulations while allowing coordinated movements. Synovial structures of the elbow include a lining the capsule and producing fluid for nourishment and lubrication, separated by anterior and posterior fat pads that accommodate volume changes during motion. Bursae such as the , located posteriorly over the process, and smaller antecubital bursae reduce at pressure points. The shared unites the three components, with the capsule's thin extensions defining precise anatomical boundaries around the humeroulnar, humeroradial, and radioulnar interfaces.

Muscles

The arm's musculature is divided into anterior and posterior compartments by the brachial fascia, a deep fascial layer that envelops the entire limb and extends medial and lateral intermuscular to the , creating distinct fascial boundaries that compartmentalize the muscles and facilitate their attachments while limiting expansion during contraction. These , originating from the brachial fascia, attach along the medial and lateral supracondylar ridges of the , separating the flexor muscles anteriorly from the extensors posteriorly and providing structural support for muscle origins. This compartmentalization helps organize , where fibers are arranged in parallel or pennate patterns to optimize force transmission via tendons. In the anterior compartment, the biceps brachii is a muscle composed of two heads: the long head originates from the of the , while the short head arises from the of the ; both heads converge into a common belly that inserts via a thick at the radial tuberosity and the on the of the . Its parallel fiber arrangement allows for efficient shortening and elongation, with the formation including a long intra-articular for the long head that traverses the capsule. The biceps brachii is innervated by the (C5-C6) and receives blood supply from branches of the . The brachialis, located deep to the biceps brachii, is a muscle with some pennate fiber characteristics; it originates from the distal half of the anterior and inserts on the coronoid process and . Its fibers run parallel to the for direct force application, forming a short at the insertion. Innervation is primarily from the (C5-C6), with minor contributions from the (C7), and blood supply comes from the and radial recurrent artery. The , a small muscle, originates from the of the alongside the short head of the brachii and inserts on the anteromedial surface of the humeral shaft at the mid-third. Its parallel fibers blend with surrounding without a distinct long , aiding in compact attachment. It is innervated by the (C5-C7) and supplied by the and anterior circumflex humeral artery. The posterior compartment houses the triceps brachii, a large with three heads: the long head originates from the of the ; the lateral head from the posterior above the ; and the medial head from the posterior below the . These heads unite into a common that inserts on the process of the and the elbow , with pennate fiber arrangements in the heads enhancing cross-sectional area for force generation. Innervation is via the , with specific root contributions (C6 for lateral, C7 for long, C8 for medial heads), and blood supply is from the profunda brachii artery. The anconeus, a small triangular , originates from the and inserts on the and proximal posterior . Its oblique fiber arrangement integrates with the tendon for seamless attachment. It is innervated by the (C7-C8) and receives blood supply from the posterior interosseous recurrent artery.

Nervous supply

The nervous supply of the arm primarily derives from the , a complex network of nerves originating from the anterior rami of the spinal nerves through T1. These roots emerge between the in the neck and undergo reorganization into trunks, divisions, cords, and terminal branches as they descend into the . Specifically, the five roots combine to form three trunks: the upper trunk from and , the middle trunk from C7, and the lower trunk from C8 and T1. Each trunk then splits into anterior and posterior divisions, yielding six divisions in total, which regroup to form three cords named relative to the second part of the : the (from anterior divisions of the upper and middle trunks), the (from the anterior division of the lower trunk), and the (from the posterior divisions of all three trunks). The terminal branches of the provide the primary motor and sensory innervation to the arm's compartments. The , arising from the , supplies motor innervation to the anterior compartment muscles, including the coracobrachialis, biceps brachii, and brachialis. In contrast, the , originating from the , innervates the posterior compartment muscles such as the brachii and anconeus. The , also from the , provides motor supply to the deltoid (though primarily associated with the ) and teres minor, while contributing sensory fibers to the lateral upper arm. The and ulnar nerves, formed by contributions from the lateral and medial cords () and solely from the medial cord (ulnar), primarily innervate the but also carry fibers affecting the arm's distal regions. Sensory innervation to the skin of the arm follows dermatomal patterns corresponding to the roots, primarily through C8, with T1 contributing medially. The dermatome covers the lateral aspect of the upper arm, extends to the lateral , supplies the posterior and hand, innervates the medial and hand, and T1 covers the medial upper arm. These dermatomes provide a segmental map for sensory distribution, overlapping slightly at boundaries to ensure comprehensive coverage. Autonomic innervation to the arm involves sympathetic fibers that hitchhike along the pathways. These postganglionic sympathetic fibers originate from the and join the plexus via gray rami communicantes, primarily from the upper thoracic levels (T1-T2), to regulate , , and pilomotor functions in the arm's and vessels. Parasympathetic fibers are absent in the brachial plexus, as upper limb autonomic supply is predominantly sympathetic. Motor distributions from the branches target specific muscle groups for arm movement, while sensory distributions provide and proprioceptive feedback via peripheral nerve territories. For instance, the musculocutaneous nerve's sensory branch (lateral of the ) supplies the lateral of the after piercing the coracobrachialis, whereas the radial nerve's posterior of the arm innervates the posterior and lateral of the upper arm. The axillary nerve's superior lateral of the arm covers the over the deltoid region, and the medial of the arm (from the medial cord) supplies the medial upper arm . These motor-sensory maps ensure coordinated innervation, with overlap in transitional zones to prevent isolated deficits.

Vascular supply

The arterial supply to the arm is provided primarily by the , which is the direct continuation of the distal to the lower border of the . The descends along the medial aspect of the within the arm, embedded in the medial intermuscular septum, and gives rise to key branches including the profunda brachii artery (also known as the deep brachial artery), which arises posteriorly near the lower border of teres major and courses through the to supply the posterior compartment muscles such as the triceps brachii. Additional branches from the include nutrient arteries to the and muscular branches to the anterior compartment. At the near the , the terminates by bifurcating into the radial and ulnar arteries, which continue into the . Anastomotic networks ensure redundancy in the vascular supply around the and . The around the involves branches from the subclavian and axillary arteries, such as the suprascapular, transverse , and arteries, forming a collateral circle that connects with the profunda brachii for alternative pathways if proximal occurs. At the , an arterial forms a periarticular network supplied by the anterior and posterior branches of the profunda brachii, the superior and inferior ulnar collateral arteries from the brachial, and recurrent branches from the radial and ulnar arteries, providing robust collateral circulation to the joint and surrounding structures. The venous drainage of the arm consists of superficial and deep systems that ultimately converge to form the axillary vein. The superficial veins lie in the subcutaneous tissue and include the cephalic vein, which originates from the dorsal venous network of the hand and ascends laterally along the arm to join the axillary vein; the basilic vein, which drains the medial aspect and joins the brachial veins in the arm before becoming the axillary vein; and the median cubital vein, a communicating vessel crossing the cubital fossa to connect the cephalic and basilic systems, often used for venipuncture. These superficial veins feature valves that direct flow toward the heart and receive tributaries from dorsal digital and palmar venous networks. The deep veins comprise paired venae comitantes, including the brachial veins, which accompany the brachial artery along its course in the arm and drain the deeper tissues, merging with superficial veins and continuing as the axillary vein proximal to the arm. Lymphatic drainage from the arm follows superficial and deep pathways that parallel the venous system. Superficial lymphatics collect from and subcutaneous tissues, draining laterally toward the deltopectoral (infraclavicular) nodes and medially toward the pectoral and subscapular axillary nodes, while passing through cubital ( located above the medial at the . Deep lymphatics drain muscles, bones, and joints, accompanying deep veins and arteries to the lateral (brachial) group of axillary nodes, which specifically receive from the arm's upper regions. From the axillary nodes, flows through central and apical groups before entering the subclavian lymphatic trunk and ultimately the or right lymphatic duct.

Function

Movements

The primary movements of the arm occur primarily at the glenohumeral () and joints, enabling a wide range of functions such as reaching, lifting, and manipulating objects. At the , flexion and extension allow the arm to move anteriorly and posteriorly in the , with flexion typically ranging from 0° to 180° and extension from 0° to 50–60°. and adduction facilitate lateral movements in the frontal , where extends from 0° to 160–180° and adduction returns from 0° to 30–40°. Rotational movements include internal rotation (0° to 70°) and external rotation (0° to 90°), which permit twisting of the along its longitudinal axis. At the elbow joint, flexion enables bending of the toward the upper arm, with a normal range of 0° to 150°. Supination and pronation, occurring at the proximal and distal radioulnar joints, allow rotational movements of the , typically ranging from 80° to 90° for each direction relative to the anatomical position. These motions position the hand for precise tasks like turning a doorknob (supination) or using a (pronation). Synergistic movements combine these actions for more complex motions, such as circumduction at the , where the arm describes a conical path through sequential flexion, , extension, and adduction while the proximal end remains relatively fixed. This circular trajectory enhances the arm's versatility in activities like throwing or drawing large circles. In the context of upper limb function, the arm serves as the proximal link in the kinematic chain, connecting the shoulder girdle to the forearm and hand to transmit and coordinate motions distally for efficient whole-limb actions. This serial linkage allows sequential activation from shoulder to elbow, optimizing reach and stability during everyday and athletic tasks.

Biomechanics

The biomechanics of the arm encompasses the mechanical principles that enable efficient force transmission and motion through its joints and muscles. Central to elbow function is the generation of torque, defined as the rotational force produced by a muscle, calculated using the equation \tau = F \times r, where \tau is torque, F is the muscle force, and r is the moment arm—the perpendicular distance from the muscle's line of action to the joint's axis of rotation. This relationship highlights how leverage amplifies small muscle forces into larger joint torques, particularly during flexion and extension. The brachii exemplifies and at the , with its distal insertion on the radial tuberosity providing a moment arm that peaks at approximately 4.7 cm, compared to an average of 3.7 cm across its . This configuration allows the to generate higher relative to the triceps brachii, whose peak moment arm is only 2.3 cm, due to differences in (PCSA) and geometry; overall moment-generating capacity is modeled as PCSA multiplied by the average moment arm and cosine of the pennation angle. The moment arm varies with angle, reaching a maximum for the at about 109° of flexion, optimizing for tasks like lifting. In supination, the short head of the produces a steeper slope relative to load, enhancing rotational efficiency. Load distribution in the arm during activities like lifting involves balancing compressive and forces to maintain . At the , compressive forces on the arise primarily from the deltoid's superior pull counteracted by the rotator cuff's depressive action, resulting in net joint loads that can exceed body weight during overhead lifting; for instance, analyses of daily activities show peak compressive forces up to 0.8 times body weight in forward flexion tasks. At the , forces develop from the transverse components of muscle tensions and external loads, particularly during eccentric control, with magnitudes influenced by the joint's valgus alignment and contributing to up to 20-30% of total force vectors in dynamic motions. These distributions ensure efficient while minimizing stress concentrations, as seen in simulations where humeral compression during cycles ranges from 20-50% of body weight under controlled loads. Muscle efficiency in the arm is governed by the length-tension relationship, which describes how active force output varies with length due to actin-myosin filament overlap. Maximal tension occurs at an optimal length of approximately 2.50 μm, where cross-bridge formation is maximized; deviations lead to reduced force on the ascending limb (short lengths, <1.65 μm) from double overlap and on the descending limb (long lengths, >3.65 μm) from decreased overlap. For flexors like the biceps brachii, this translates to peak isometric strength at angles of 50-60°, where muscle length is stretched about 20% beyond resting, aligning with broader force-length curves observed in whole-muscle models. output, the product of and , peaks during flexion at intermediate loads (around 50% of maximum strength), where maximal shortening reaches 300-500°/s, enabling efficient dynamic performance in ballistic movements. Ergonomic factors in arm emphasize angles that maximize strength while minimizing fatigue, particularly for repetitive tasks. Maximum flexor torque occurs at flexion angles of 56°, where the operates near its length-tension optimum, generating up to 10-15% higher force than at 90°; conversely, triceps extension strength peaks at 84°. These angles inform designs, such as positioning loads at 90-110° for balanced leverage, reducing and enhancing by aligning arms with muscle force vectors.

Development

Embryology

The development of the human begins in the fourth week of embryonic , when limb buds emerge as paddle-like outgrowths from the lateral body wall. These buds arise from the interaction between the and overlying , with mesenchymal cells in the layer of the proliferating to form the core of the bud around day 26 of development. The bud appears slightly before the lower limb bud, positioned at the levels of C5 to T1. Crucial to this process is the formation of the apical ectodermal ridge (AER), a thickened ectodermal structure at the distal tip of the limb bud that secretes fibroblast growth factors (FGFs), particularly FGF8, to promote proximal-distal outgrowth and patterning. Complementing the AER is the zone of polarizing activity (ZPA), a region of mesenchyme at the posterior margin of the limb bud that expresses Sonic hedgehog (Shh), directing anterior-posterior patterning through a that specifies digit identity and overall limb asymmetry. Muscles and bones of the derive from distinct embryonic origins, patterned by expression. Skeletal muscles originate from myotomal cells of the somites, which are segmental blocks of paraxial ; these myoblasts migrate into the limb bud via the myogenic lineage, differentiating under the influence of signals like and to form the flexor and extensor compartments. In contrast, the bones and connective tissues arise from the within the limb bud, where sclerotome-like precursors contribute minimally to limb in humans, unlike in lower vertebrates; instead, such as HoxA and HoxD clusters regulate the segmentation and identity of these mesenchymal condensations, which chondrify and ossify to form the , , , and hand elements. Neural and vascular elements develop concurrently with limb outgrowth to support the expanding bud. The forms from the ventral rami of spinal nerves to T1, with axonal outgrowth beginning in the fourth week as motor neurons extend processes into the limb , guided by cues like netrins and semaphorins to innervate emerging muscle groups. Vascular supply initiates via the axis artery, an embryonic vessel derived from the 7th cervical intersegmental artery and contributions from the dorsal aorta, which penetrates the limb bud early; proximal branches connect to the (notably the left 4th arch forming part of the system), while remodeling establishes the subclavian, axillary, and brachial arteries by the eighth week. A notable teratogenic during embryogenesis is exposure to , a drug marketed in the late 1950s and early 1960s for , which caused severe limb reductions like in thousands of infants when taken between days 20 and 36 post-fertilization. disrupts by inhibiting cereblon-mediated degradation of transcription factors and interfering with FGF and Shh signaling in the limb bud, leading to halted outgrowth and malformed AER function; this tragedy, affecting over 10,000 births worldwide before the drug's withdrawal in 1961, underscored the vulnerability of weeks 4 to 8.

Postnatal changes

Following birth, the arm undergoes continued and growth, building on prenatal foundations. The primary of the appears at approximately 8 weeks of , but postnatal involves the and of secondary centers. For the proximal ( region), the head ossifies between 1 and 6 months of age, the at around 1 year, and the between 3 and 5 years. At the distal ( region), secondary centers include the capitellum (2-24 months), medial (4-7 years), trochlea (8-10 years), lateral (10-13 years), and (8-10 years), with generally completing by late . During childhood and puberty, the arm experiences significant growth spurts, particularly in length and strength, influenced by hormonal changes. Pubertal growth accelerates arm bone elongation, with males exhibiting a more pronounced spurt due to higher testosterone levels, leading to sexual dimorphism where adult male humerus length averages about 30.5 cm compared to 27.7 cm in females—a difference of roughly 10%. Muscle strength in the arm also increases more substantially in males during this period, enhancing overall upper limb power. These changes contribute to adult arm proportions, with full arm length (from shoulder to fingertip) typically reaching 70-80 cm in males and 65-75 cm in females. In later life, age-related alterations affect arm structure and function. leads to progressive , reducing arm muscle mass by 1-2% annually after age 50, resulting in decreased strength and grip force. Concurrently, declines, with postmenopausal women at higher risk for , potentially losing 20-30% of trabecular bone in the over decades, increasing fracture susceptibility. These changes are universal but vary by lifestyle and health factors. Ethnic and geographic variations influence average arm dimensions, reflecting genetic and environmental factors. For instance, adult (a for arm length) averages around 180 cm in European populations, 170 cm in East Asian groups, and 175 cm in African cohorts, with lengths similarly varying (e.g., 31-32 cm in Caucasians vs. 29-30 cm in South Asians). These differences, typically 5-10% across groups, underscore the need for population-specific anthropometric data in clinical and ergonomic applications.

Clinical significance

Injuries

Injuries to the arm encompass a range of traumatic conditions primarily involving bone fractures and damage, often resulting from high-impact events that disrupt normal anatomical integrity. These acute traumas can lead to significant , swelling, and impaired , necessitating and to prevent complications such as neurovascular compromise. Common injury patterns include fractures of the and dislocations, alongside disruptions like muscle strains and sprains, which are frequently managed through conservative measures in the initial phase. Fractures of the humerus, particularly mid-shaft variants, represent a prevalent traumatic injury, accounting for approximately 3-5% of all fractures and often resulting from direct blows to the arm or torsional forces during falls. These injuries are more common in adults following high-energy trauma, such as motor vehicle accidents or assaults, and may be associated with radial nerve involvement due to the nerve's proximity along the bone's spiral groove. Elbow dislocations, the most frequent large-joint dislocation after the shoulder, occur in about 6 per 100,000 individuals annually, with posterior dislocations comprising 90% of cases due to hyperextension forces that drive the olecranon into the humerus. These dislocations can involve associated fractures of the radial head or coronoid process, heightening the risk of instability if not addressed acutely. Soft tissue injuries, including muscle strains and ligament sprains, frequently accompany or occur independently of bony in the arm. Distal tendon ruptures, a notable muscle strain, predominantly affect males aged 40-60 years during eccentric loading activities like , leading to a characteristic "" deformity from proximal retraction of the muscle belly. Ligament sprains, such as those involving the at the or acromioclavicular ligaments at the , result from excessive valgus stress or rotational forces, graded from mild stretching (grade 1) to partial tears (grade 2) based on fiber disruption. Trauma mechanisms for arm injuries typically involve falls on an outstretched hand (FOOSH), which transmits axial loads through the to the and , commonly causing distal fractures, elbow dislocations, or proximal humeral impacts in older adults. Direct blows, such as those from sports collisions or assaults, more often produce mid-shaft humeral fractures by applying transverse or compressive forces to the bone. Initial management of these injuries emphasizes the protocol—rest to avoid further damage, to reduce inflammation, compression to minimize swelling, and elevation above heart level to promote venous return—applied for the first 48-72 hours post-injury. For humeral shaft fractures, immobilization with a or functional supports alignment and allows early motion, typically for 3-6 weeks until formation. dislocations require urgent closed reduction under sedation to restore congruity, followed by immobilization for 1-3 weeks to protect healing while monitoring for recurrent instability. injuries like ruptures or sprains similarly benefit from and brief support, with referral for surgical repair if complete tendon disruption is confirmed.

Diseases

Diseases of the arm encompass a range of non-traumatic pathological conditions that impair function through , , or vascular compromise, often arising from repetitive use, positional factors, or idiopathic mechanisms. These disorders can lead to pain, weakness, and sensory deficits, necessitating prompt to prevent irreversible damage. Common examples include compartment syndromes, neuropathies, vascular thromboses, and inflammatory conditions affecting bursae and tendons. Compartment syndrome in the upper arm involves elevated intracompartmental within the fascial envelopes surrounding muscle groups, compromising and leading to ischemia if untreated. The arm has distinct anterior and posterior compartments; the anterior contains the brachii and brachialis muscles, while the posterior includes the brachii. Diagnosis relies on clinical signs such as disproportionate pain, , and passive stretch pain, confirmed by intracompartmental exceeding 30 mmHg, which indicates tissue hypoperfusion relative to diastolic . is indicated for pressures above this threshold or when delta pressure (diastolic minus compartment ) falls below 30 mmHg, aiming to release fascial constraints and restore blood flow before occurs. Neuropathies affecting the arm primarily involve compression of major nerves, leading to motor and sensory impairments. palsy, often termed Saturday night palsy, results from prolonged compression of the at the spiral groove of the , typically during sleep or , causing axillary nerve-sparing , finger extension weakness, and sensory loss over the dorsal hand. Symptoms include radial deviation of the wrist and inability to extend the metacarpophalangeal joints, with recovery often occurring spontaneously over weeks to months due to its neuropraxic nature. issues at the arm level are rarer and may stem from compression proximal to the , such as in pronator teres syndrome variants or iatrogenic causes, presenting with , thenar weakness, and paresthesias in the median distribution without signs. Electrodiagnostic studies help differentiate these from distal entrapments. Vascular disorders in the arm, such as , are uncommon and frequently associated with repetitive microtrauma rather than acute injury, leading to or with resultant ischemia. In overhead athletes like pitchers, the artery may undergo intimal from cyclic compression between the humeral head and during throwing motions, predisposing to formation. Symptoms include acute , , pulselessness, and coolness in the distal arm, potentially progressing to tissue loss if untreated, though isolated arm involvement remains rare outside athletic contexts. Inflammatory conditions of the arm often target periarticular structures, with subacromial bursitis causing inflammation of the subacromial-subdeltoid bursa due to repetitive overhead activities or minor impingements, resulting in anterior shoulder pain exacerbated by abduction. This shoulder-adjacent bursitis leads to swelling, tenderness, and restricted motion, commonly coexisting with rotator cuff irritation. Tendonitis, particularly of the long head of the biceps brachii, involves inflammation at its supraglenoid origin, provoked by overuse in activities like weightlifting, manifesting as deep anterior arm ache, bicipital groove tenderness, and pain on resisted supination or flexion. These conditions highlight the arm's vulnerability to cumulative stress on tendinous and bursal tissues.

Comparative anatomy

In other animals

In quadrupeds such as and , the serves primarily as a structure, consisting of the , , , and , with adaptations that enhance stability and load distribution during locomotion. In , the , , and form a robust that efficiently supports body weight, often with the and partially fused or closely apposed for added strength in terrestrial movement. Cats exhibit a broad, flat that provides extensive attachment sites for muscles, enabling powerful propulsion, while their forelimbs maintain the core bones of , , and tailored for agile, quadrupedal gait. Among primates, the demonstrates elongation suited for brachiation, with an intermembral index of approximately 110—indicating forelimbs about 10% longer than hindlimbs—compared to the index of around 67, reflecting relatively shorter in humans. This proportion includes a longer relative to body size, facilitating suspension and swinging through trees, where the arm acts as a pendulum-like during arboreal travel. In birds and bats, forelimb modifications support flight through skeletal fusions and elongations. Bird wings derive from the tetrapod forelimb, featuring a humerus connected to a radius and ulna that support the mid-wing, with distal bones fused into a carpometacarpus for rigidity and feather anchorage during aerial propulsion. Bat wings, as modified mammalian forelimbs, elongate the humerus and dramatically extend the digits to span a patagium membrane, with some carpals fused to optimize lightweight structure for powered flight, though lacking the extensive proximal fusions seen in avian wings. Reptilian forelimbs, such as those in lizards, retain a more generalized tetrapod configuration with distinct humerus, radius, and ulna, but in flying forms like pterosaurs (extinct reptiles), similar elongations and reductions occur for wing support. Marine mammals like whales and dolphins have transformed their forelimbs into flippers for hydrodynamic control, with the , , and embedded in a paddle-like structure featuring an immobile to streamline maneuvering and reduce during . These adaptations prioritize steering over weight-bearing, contrasting the human arm's manipulative versatility, and include shortened, flattened bones encased in for efficient aquatic locomotion.

Evolutionary aspects

The evolutionary history of the arm traces back to the origins of tetrapods approximately 400 million years ago during the period, when sarcopterygian fish—lobe-finned vertebrates—underwent a pivotal transition from aquatic to terrestrial environments. In this shift, the pectoral fins of these fish, supported by robust bony elements, evolved into the foundational structure of forelimbs capable of supporting body weight on land. This transformation enabled early amphibians like to propel themselves on shallow substrates, marking the emergence of the polydactyl limb plan with a , , , and digit-like rays that prefigured the arm's modular architecture. In the lineage leading to primates, arm evolution accelerated approximately 66 million years ago during the late , as small, insectivorous mammals adapted to arboreal lifestyles in forested environments. These early , such as , developed elongated forelimbs with enhanced joint flexibility for grasping branches and leaping between trees, reflecting selection for precise manipulation over quadrupedal pronogrady. A key adaptation was the reconfiguration of the , which became more dorsally oriented and laterally positioned relative to the , facilitating overhead arm and increasing glenohumeral mobility for below-branch locomotion. This scapular shift, evident in fossils like those of Adapiformes, enhanced rotational freedom at the , distinguishing primate arms from those of other mammals and supporting the order's hallmark . Human arm evolution diverged markedly with the advent of habitual in early hominins around 4 million years ago, exemplified by species, which redistributed locomotor demands to the lower limbs and alleviated stress on the upper body. This postural change allowed the arms to specialize for reaching and handling rather than propulsion, with reduced humeral robusticity and elongated forearms adapting to overhead activities while retaining some arboreal capability. Concurrently, the selective pressures of tool use—evident from implements dating to about 2.6 million years ago but inferred earlier—favored refinements in the precision grip, involving a longer, more opposable and stabilized metacarpophalangeal joints for fine in stone and object manipulation. Fossil evidence from , dated to 4.4 million years ago in Ethiopia's , illuminates this transitional phase through partial arm skeletons, including a that exhibits a mediolaterally compressed shaft and a shallow indicative of both suspensory climbing and emerging terrestrial dexterity. The , shorter and more robust than in later australopiths, suggests retention of chimpanzee-like arboreal traits alongside modifications for bipedal posture, such as a reduced deltopectoral crest for less reliance on arm swing during walking. These specimens, from the ARA-VP-7/50 skeleton, underscore how early arms balanced climbing efficiency with the nascent demands of upright and manual skill, bridging suspension to manipulation.

Society and culture

Symbolism and art

The raised arm has long served as a powerful iconographic motif in and , often denoting authority, allegiance, or triumph. Although popularly linked to ancient practices—such as the supposed "" depicted in lictors carrying bundles—the gesture's historical origins are debated, with no definitive evidence from classical sources confirming its use as a formal in . Instead, the outstretched arm gained prominence in modern times through fascist regimes; adopted it in the as a of imperial revival, drawing on romanticized notions of grandeur, while Hitler's Nazi regime formalized it as the "Sieg Heil" to enforce ideological conformity from 1933 onward. Following , the Nazi variant was banned in under the 1945 Law No. 39 and subsequent penal code provisions, criminalizing its public display as Nazi with penalties up to three years imprisonment, a restriction echoed in several nations to prevent the resurgence of far-right . In , the arm's dynamic pose often embodied victory and human potential, as seen in Michelangelo's (1501–1504), where the figure's tensed right arm holds a behind his in a moment of poised readiness before battle, symbolizing Florence's defiant republican spirit against tyranny rather than post-victory celebration. This stance, with veins bulging and muscles flexed, highlighted anatomical precision to convey intellectual resolve and moral strength, influencing subsequent depictions of heroic figures in Western sculpture. The arm has frequently appeared as a prominent canvas for tattoos and adornments, signifying identity, status, and . In Polynesian societies, tribal tattoos known as tatau—elaborate geometric patterns covering arms and torsos—originated over 2,000 years ago as rites of , encoding genealogical histories, social rank, and spiritual protections through motifs like sharks for warriors or waves for navigators, with the process itself testing endurance and community bonds. sailors encountered these traditions during 18th-century voyages to the Pacific, adopting arm tattoos as protective talismans and markers of ; by the late 1700s, approximately one-third of British personnel bore such ink, often anchors for stability or swallows for safe return, blending superstition with personal narrative. In mythology, the arm symbolizes superhuman prowess and endurance, most iconically through ( in lore), whose twelve labors—from strangling the to capturing the —relied on his Herculean arms, depicted in vase paintings and mosaics as massively muscled limbs wielding a club, emblematic of overcoming chaos and asserting divine favor. Norse folklore similarly features prosthetic limbs in sagas, portraying them not as diminishment but as enhancements of heroic agency; for instance, the 10th-century warrior Ǫnundr Tree-Foot, referenced in Icelandic family sagas, crafted wooden limb replacements after injury, continuing raids and embodying resilience, while fantastical variants in legends granted magical enhancements to maintain societal roles. Muscular arms have reinforced gender symbolism, particularly tropes of masculinity in 19th-century , where emerged as a response to industrialization's emasculating effects. The "muscular Christianity" movement, promoted by figures like , idealized brawny arms as signs of moral vigor and imperial duty, inspiring regimens that equated physical power with Christian manhood. Pioneers such as , dubbed the "father of modern " in the 1890s, flexed veined, hypertrophied arms in performances and publications, transforming them into emblems of scientific self-improvement and racial superiority, influencing global standards of male virility.

Sports and activities

In , the sport emphasizes the arm's rotational strength, particularly through supination of the during techniques like the "hook," where competitors apply by rotating the palm upward to pin the opponent's hand. This movement relies heavily on the brachii and supinator muscles to generate against resistance, often resulting in high loads on the . Injury risks are significant, with spiral fractures of the distal humeral shaft being the most common due to the torsional forces exceeding strength, especially in untrained individuals or during sudden application. World records in specialized lifts, such as pronation curls, demonstrate exceptional arm power, with athletes like achieving over 80 kg (176 lbs) in one-arm pronation lifts, surpassing the 100 lb threshold in training feats that highlight supinatory strength. Weightlifting exercises prominently feature the arm for targeted muscle development and compound movements. Bicep curls isolate the brachii by flexing the against resistance, typically using or dumbbells to build peak contraction and hypertrophy in the upper arm. Overhead presses, conversely, engage the triceps brachii and deltoids through elbow extension, pressing weights from shoulder height to full arm lockout, which enhances overall arm and pushing power. In , the clean—a foundational lift—involves explosive arm pull and catch phases to hoist the from the floor to the shoulders, with intermediate male athletes commonly achieving 100 kg cleans as a for competitive readiness in lighter weight classes. Martial arts disciplines utilize the for both defensive and offensive maneuvers, integrating precise blocks and strikes that demand rapid muscle activation. In like the gedan barai (low block) employ pronation to deflect low attacks, absorbing impact through the and while maintaining structural alignment to protect the body. strikes, such as jabs and hooks, leverage arm extension and rotation for , with the lead arm delivering straight-line force via contraction and the rear arm generating through and synergy. methods include knuckle push-ups and heavy bag drills to toughen extensors and flexors, improving impact resistance and endurance for sustained arm usage in . Recreational activities like heavily engage the arm through sustained grips that extend function to full coordination. Climbers use crimp and open-hand grips, where finger flexors in the contract isometrically to hold body weight, while extensors counterbalance to prevent flexion and fatigue. This -dominant effort transitions into broader arm pulling, activating the latissimus dorsi and to mantle or dyno up routes, emphasizing how directly supports arm-driven and on vertical .